Biomedical electrode configuration for suppressing movement artifact

ABSTRACT

A biomedical sensor for detecting EMG signals may include a non-conducting fixed framework supporting a symmetrical arrangement of four electrode surfaces, configured as two signal detection contacts, each with a respective associated signal reference contact. The mechanical and electrical configuration act together to suppress movement artifact.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional App. No.61/271,930, filed Jul. 28, 2009, which is incorporated herein byreference.

TECHNICAL FIELD

This application relates to the field of sensing bio-potentialsgenerated within a living body and, more particularly, to sensors placedon the surface of the skin for detecting the electrical activity frommuscles.

BACKGROUND OF THE INVENTION

Depolarization potentials created during a muscle fiber contractiongenerate an electrical field gradient which propagates in a directionalong the fibers throughout the volume conductor comprised of themuscle, the surrounding tissue, and skin layers. Electrodes placed onthe skins surface allow for the non-invasive detection of thiselectrical field gradient providing the temporal summation of thepropagating depolarization potentials of the active muscle fibers in theunderlying vicinity of the electrode. The resulting voltage on theskin's surface is termed the surface electromyographic signal (sEMG).

In order to measure this voltage, an electrolytic interface may beformed between the electrolytes in the subcutaneous tissue and the ohmicconductive surface of the electrode contact attached to the skinssurface. The primary electrical conduit between the subcutaneous volumeconductor and the skins surface is established via the sweat ducts whichpass through the non-conductive stratum corneum so that sweat andmoisture from the underlying sweat glands are deposited onto the skinssurface completing the electrolytic interface.

The electrolytic interface is provided by disassociated ions from theelectrolyte forming a layer on the conductive electrode contact surface(Nernst polarization or contact half-cell potential). Depending on thechemical composition, concentration of the electrolytes on the skin, andthe composition of the electrode contact metal, the half-cell potentialscan range in amplitude to several hundred millivolts. Signal potentialsemanating from the muscle in the underlying tissue are conveyed viaionic transport through the electrolyte to the exposed conductivecontact surface of the electrode. The signal amplitude is typicallyseveral orders of magnitude smaller than the half-cell potential andranges from 10 microvolts to 5 millivolts. The resultant voltage sensedby the electrode contact is therefore the electrical summation of thesignal potential and contact half-cell potential.

When the electrolytic skin interface of the electrode is mechanicallydisturbed due to relative movement or pressure changes between thetissue and conductive surface of the electrode, the effectiveconcentration of the electrolytes can be altered so that the resultanthalf-cell potential amplitude is modulated by the mechanicaldisturbance. The modulation of hall-cell potential is termed “movementartifact” and typically arises from rapid body movements, or objects orclothing coming into contact with the sensor case housing theelectrodes.

Movement artifact can be particularly problematic as the change inhalf-cell potential can exhibit large (>50 mV) voltage deviations whichoverwhelm the amplitude of the sEMG signal. An additional source ofmovement artifact is due to the triboelectric charge that can accumulateon the non-conducting stratum corneum as a result of walking on carpetor contact with certain fabrics under low humidity conditions. Thiseffect can be especially problematic when the electrolytic skininterface exhibits high impedance resulting from the lack of suitablemoisture between the electrode contact and the skin. This impedance canreach tens of megohms for contacts with an area of 1 mm squared placedon unprepared skin.

Prior art techniques developed to address reduction in movement artifactfocus on the materials used to fabricate the conductive surface of theelectrode contact. Highly conductive aqueous salt solutions orhydrophilic gels applied to the contacts surface act to improve theelectrolytic skin interface by augmenting the nature moisture present onthe skins surface to stabilize and reduce skin impedance, however thesematerials can create large half-cell potentials, and if not properlyapplied, can leak to form low impedance bridges between electrodecontacts thereby “shorting out” the desired bio-potential signal.Electrodes formed as an insulating capacitive plate overcome some of theproblems associated with half-cell potential created when using ohmicelectrodes, but are subject to the triboelectric charge effect on drystratum corneum, and therefore subject to movement artifact and staticdischarge when placed under clothing. The class of “dry” electrodesformed from silver metal or silver coated plastic contacts falls inbetween the two extremes of gel and capacitive electrode designs. Theyhave higher initial impedance than gel, relying on the inherent moisturepresent on the skin to form the electrolytic interface. They aretypically formed of pure silver or silver-silver chloride.

Regardless of the contact materials used, it is known that theconfiguration of a sensor designed to detect sEMG signals may includetwo disposable electrode contact discs, one for each signal input placedsingularly, or in pairs, mounted on a flexible non-conductive adhesivepad so that the applied contact conforms to the underlying body surface.The electrodes may be attached by snaps or spring loaded clips andconnected to remote electronic circuitry via a shielded insulatedtethered cable.

Characteristically, the sensor includes the two signal contacts locatedover the muscle and a third “reference” contact located at anelectrically inactive location on the body. In some sensors the twosignal and reference contacts are placed on the same insulating pad inthe form of an equilateral triangle. The orientation for the signalinput contact pair may be in a direction parallel to the muscle fiber.The recording configuration may be the differential configuration wherethe voltage at each signal input contact is measured with respect thethird reference contact and subtracted using a differentialpre-amplifier circuit. In this way, any voltages common to bothelectrodes such as half-cell potentials and line interferenceeffectively subtract to zero for an ideal amplifier. However, incompliant or flexible electrode skin interface designs, disturbances tothe electrode interface induced from contact forces applied directly tothe interface or induced from shear forces applied to the skin arelikely to cause an unequal localized disruption of the electrolytejunction half-cell potential of each electrode contact. This unequalchange in half-cell potentials can not be removed by differentialsubtraction and as a result generates a movement artifact signal.

As further background to the system described herein, sEMG sensors withor without onboard signal conditioning circuitry are typically tetheredvia a cable which acts to power and convey the output signal from thesensor to external instrumentation. With the advent of wirelesstechnology, wireless versions of sEMG sensors are becoming analternative to tethered sensors. These sensors typically dispense withthe reference ground contact due to the fact that unlike tethereddesigns, they electrically “float” with respect to earth ground. Theseare termed “reference free” designs. The two electrode sensor can beconfigured either as a mono-polar, or as a differential amplifier withan internally generated electrical reference. Both configurationsmeasure the voltage difference measured between the two contacts. Atypical wireless sensor consists of two gel filled disk contacts mountedon disposable adhesive backed pad. Snap leads connect the electrodes viaa short cable to the inputs of a wireless module containing the sEMGsignal conditioning electronics.

All of the aforementioned electrode contact and sensor configurationsdescribed as prior art, whether tethered or wireless, are subject to theeffects of movement induced artifact, due to inability to electricallystabilize the electrolytic interface during a mechanical disturbance tothe electrode contacts. The commonly-applied technique to mitigatesensitivity to movement artifact is to decrease and stabilize theimpedance of the skin-electrode interface using conductive aqueous saltsolutions or hydrophilic gels applied to disposable electrodes. Whencombined with a differential recording configuration using a remotereference, these solutions are partially effective but incur theproblems associated with application of pastes or dehydration of gelsover time, limiting their useful self-life.

The problems of artifact and sensitivity to electro-static fields areespecially severe for wireless “reference free” sEMG sensor designswhich use only two contacts. Wireless solutions utilizing differentialrecording from a triangular configuration of three disposableelectrodes, while less affected by electro-static fields, are stillsubject to movement induced artifacts.

Accordingly, it would be desirable to provide an electrode configurationwhich when applied to unprepared skin, can suitably detect musclesignals during highly dynamic activities, while suppressing associatedmovement artifact. Furthermore, the electrode configuration would bedirectly applicable to wireless sEMG sensor technologies which providethe inherent benefit of un-tethered, unencumbered measurement frommuscles during these types of activities.

SUMMARY OF THE INVENTION

According to the system described herein, a biomedical sensor includes asubstrate and a plurality of conductive areas arranged on the substrateand configured as signal detection contacts each with a respectiveassociated signal reference contact, forming a plurality of signaldetection/reference contact pairs. A differential sensing circuit isincluded, in which each signal detection/reference contact pair reactsin response to an electrical artifact manifestation of an appliedmechanical disturbance, and wherein at least one common electricalcomponent of the artifact measured by the plurality of signaldetection/reference contact pairs is cancelled out by the differentialsensing circuit. The substrate may be rigid.

A fixed framework may be coupled to the substrate, in which thesubstrate is a body-directed electrically insulating bottom surface ofthe framework, forming an electrode to body interface. A symmetricalarrangement of two signal input and two signal reference contacts may beretained by the fixed framework, each having a contact surfaceprojecting from the bottom surface and a coupling surface projectingabove the top surface. The system described herein relates to a type ofelectrode configuration which can suppress movement induced electricalartifacts. The system described herein is based on the concept thatunlike existing compliant designs typical of disposable electrodes, afixed, non-compliant framework supporting a symmetrical arrangement ofconductive areas provides for the mechanical stabilization of theunderlying tissue.

The four conductive areas, when configured as two signal detectioncontacts, each with a respective associated signal reference contact innearby proximity, may react similarly and in unison to the electricalartifact manifestation of an applied mechanical disturbance in such away that the resulting common electrical components of the artifact canbe canceled out by a differential sensing circuit configuration. Thefour conductive areas may be disposed and formed according to variousembodiments of the system described herein. The symmetrical arrangementof the contacts may be configured as two signal detection contacts, eachwith a respective associated signal reference contact in nearbyproximity, forming two signal/reference contact pairs. Thesignal/reference contact pairs may form two co-lineardetection/reference contact pairs oriented in parallel with respect toeach other and spaced apart so that the detection contact is aligned tobe on the same side of each pair.

Furthermore, each of the four conductive areas may be formed in theshape of a conductive rod aligned in parallel on the plane of thebody-directed insulating substrate, and projecting from thebody-directed fixed substrate. The body-directed insulating bottomsurface substrate may be contoured to surround each of the fourprojecting conductive areas. Each of the four conductive areas may beformed in the shape of a disc attached to and coplanar with thebody-directed insulating bottom surface substrate and/or may be formedin the shape of a dome whose cross-sectional profile exhibits a curvedconcave shape attached to and projecting from the body-directedinsulating bottom surface substrate. Each of the four conductive contactareas may be formed of substantially identical material composition,size and shape. Each of the four conductive areas may be formed from anoble metal and/or may be formed from a conductive plastic and/or eachof the four conductive areas may be formed from a conductive regionmatrix integrated within the matrix of the insulating substrate.

The insulating bottom surface substrate may be composed ofnon-conductive, non-polarizable, low triboelectric, bio-compatiblematerials. The insulating bottom surface substrate may be attached tothe skin by means of a double-sided adhesive membrane sheet with cut-outareas aligned with the perimeter of each of the four conductive areas.

The fixed framework may include a protective enclosure withmulti-conductor cable having individual insulated conductorsinterconnected with the coupling surface of the contacts so that onlythe conductive areas are exposed. The protective enclosure may retain awireless transceiver, and power supply, and differential amplifierinterconnected with the coupling surface of the contacts so that onlythe conductive areas are exposed. The enclosure may include anindicating marker aligned in parallel with the preferred orientation andof the sensor with respect to the direction of signal propagation anddelineating the location of the signal input contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the system described herein will be explained in detailbelow according to the figures, which are briefly described as follows:

FIG. 1 is an elevation view of a biomedical sensor according to anembodiment of the system described herein;

FIG. 2 is a top view of the case component of the sensor shown in FIG.1;

FIG. 3 is a bottom view of fixed framework substrate retaining contactsshown in FIG. 1;

FIG. 4 is a perspective view of the double sided adhesive interface usedwith the sensor of FIG. 1;

FIG. 5 is a perspective view of the double sided adhesive interfacepositioned for attachment to the sensor of FIG. 1;

FIG. 6 is a schematic top view of the sensor of FIG. 1 mounted adjacentto a bundle of muscle fibers;

FIG. 7 is a schematic side view of the sensor and muscle fiber bundleshown in FIG. 6;

FIG. 8 is a plan view of another contact embodiment of the systemdescribed herein;

FIG. 9 is a partial cross-sectional view of the contact shown in FIG. 8;

FIG. 10 is a partial cross-sectional view of an alternate form of thecontact shown in FIG. 9;

FIG. 11 is a perspective view of the double sided adhesive interfaceused with the electrodes of FIG. 8

FIG. 12 is a perspective view of another biomedical sensor according toanother embodiment of the system described herein;

FIG. 13 is a side view of the contact used with the sensor of FIG. 12;

FIG. 14 is an assembly view of the sensor of FIG. 12 and adhesiveinterface shown in FIG. 4;

FIG. 15 is a schematic diagram of the differential amplifier recordingconfiguration of the sensor of FIG. 12; and

FIGS. 16A-C show a comparison of induced movement artifact of the sensorof FIG. 12 with respect to a comparable wireless 2-bar “reference-free”sensor electrode configuration under various conditions.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

A biomedical sensor 11 includes a molded case 41 that retains apreamplifier and associated electronics 42 for wireless transmission ofthe detected signal as illustrated in FIG. 1. Disposed on the bottomsurface 21 of the case 41, or other type of framework, may be a rigidfixed substrate surface 44. Disposed on the substrate surface 44 of thecase 41 (FIG. 3) may be a symmetrical, parallel array of cylindrical barcontacts 16 each retained by a projecting portion 15 electricallyinterconnected with the preamplifier 42. Included among the contacts 16are two pairs of contacts 17, 18. One pair of contacts 17 may include apositive signal detection contact 25 and a reference contact 27. Asecond pair of contacts 18 may include a negative signal detectioncontact 26 and a reference contact 28. The two signal detection contacts25, 26 and the two reference contacts 27, 28 may be linearly alignedwith respect to the case indicia 51 disposed on the top surface 53 ofthe case 41 illustrated in FIG. 2, and arranged so that that one contactpair 17 is positioned with respect to the tail 46 of the arrow 52 andone contact pair 18 is positioned with respect to the head 47 of thearrow 52. The contact pairs 17, 18 may be further arranged so that thesignal detection contacts 25, 26 are linearly aligned with respect toeach other and oriented in parallel to the axis of the arrow 52.Similarly, the reference contacts 27, 28 may be linearly aligned withrespect to each other and oriented in parallel to the axis of the arrow52. Each of the cylindrical contacts 16 may be formed from a suitableelectrically conductive material with an exposed contact surface 22 ofequal area.

Also included in the sensor embodiment 11 may be an attachment interface156 shown in FIG. 4 having an adhesive interface surface 157 forsecuring the interface 156 to the case 41 and an adhesive interfacesurface 158 for securing the case 41 to the skin 61 of the subject. Theinterface 156 defines a plurality of apertures 159 in the form of slotsdisposed to receive the contacts 16. Tab 91 may be attached to one end92 of the top surface 157 of the interface 156.

Prior to use of the sensor 11, the top adhesive surface 157 of theattachment interface 156 may be positioned (FIG. 5) so that eachaperture 159 is aligned and centered with its respective contact 16 andthen attached to the bottom surface 21 of the case 41. Next, theadhesive bottom surface 158 of the attachment interface 156 may beadhered to the skin 61 of the subject using the arrow 52 to orient thesensor 11 with respect to the muscle fibers 55. Proper orientationaligns the arrow 52 with the fibers 55 of the bundle 54 and pointed in adirection from the proximal end 57 toward the distal end 58 of thebundle 54 as shown in FIGS. 6 and 7. The proximal end 57 may be locatedadjacent to an innervation zone 60 established at the terminal portionof the nerve 59. That arrangement of the sensor 11 may establish a knownpositive potential for the detector contact 25 and a negative potentialfor the detector contact 26.

Another biomedical sensor embodiment 145 is illustrated in FIGS. 8, 9,and 10. Disposed on the bottom surface 21 of the case 41 may be a rigidfixed substrate surface 44. Disposed on the substrate surface 44 of thecase 41 (FIG. 3) may be a symmetrical, parallel array of disc contacts23 each retained by a projecting portion 15 electrically interconnectedwith the preamplifier 42. Included among the contacts 23 are two pairsof flat disc contacts 17, 18. One pair of contacts 17 may include apositive signal detection contact 124 and a reference contact 126. Asecond pair of contacts 18 may include a negative signal detectioncontact 125 and a reference contact 127. The two signal detectioncontacts 124, 125 and the two reference contacts 126, 127 may belinearly aligned with respect to the case indicia 51 disposed on the topsurface 53 of the case 41 illustrated in FIG. 2, and arranged so thatthat one contact pair 17 is positioned with respect to the tail 46 ofthe arrow 52 and one contact pair 18 is positioned with respect to thehead 47 of the arrow 52.

The contact pairs 17, 18 may be further arranged so that the signaldetection contacts 124, 125 are linearly aligned with respect to eachother and oriented in parallel to the axis of the arrow 52. Similarly,the reference contacts 126, 127 may be linearly aligned with respect toeach other and oriented in parallel to the axis of the arrow 52. Each ofthe disc contacts 23 may be formed from a suitable electricallyconductive material with an exposed contact surface 24 of equal area. Analternative form of the disc contacts 23 is shown in FIG. 10. Thecontact surface 24 of each contact 134, 135, 136, 137 may be shaped inthe form of dome with a curved concave cross-sectional profile 130. Theprofile 130 may form a catenary curve.

Also included in the sensor embodiment 145 may be an attachmentinterface 154 shown in FIG. 11 having an adhesive interface surface 157for securing the interface 156 to the case 41 and an adhesive interfacesurface 158 for securing the case 41 to the skin 61 of the subject. Theinterface 154 defines a plurality of apertures 155 in the form ofcircles disposed to receive the contacts 23. Tab 91 is attached to oneend 92 of the top surface 157 of the interface 156. The sensorembodiment 145 may be used in a manner similar to that described abovefor the embodiment 11.

FIGS. 12 and 13 illustrate another biomedical sensor embodiment 134. Aninjection molded case 41 may retain a preamplifier and associatedelectronics 42 for wireless transmission of the detected signal asillustrated in FIG. 13. Disposed on the bottom surface 221 of the case41 may be a rigid fixed substrate surface 44. Disposed on the substratesurface 44 of the case 41 (FIG. 12) may be a symmetrical, parallel arrayof elongated cylindrical bar contacts 138 retained by the rigid fixedsubstrate surface 44 and projecting outwardly from the bottom surface 53of the case 41. An inwardly projecting portion 15 may electricallyinterconnect the contacts 138 with the preamplifier 42. Included amongthe contacts 138 may be two pairs of contacts 117, 118. One pair ofcontacts 117 may include a positive signal detection contact 225 and areference contact 227. A second pair of contacts 118 may include anegative signal detection contact 226 and a reference contact 228. Thetwo signal detection contacts 225, 226 and the two reference contacts227, 228 may be linearly aligned with respect to the case indicia 51disposed on the top surface 53 of the case 41 illustrated in FIG. 2, andarranged so that that one contact pair 117 is positioned with respect tothe tail 46 of the arrow 52 and one contact pair 118 is positioned withrespect to the head 47 of the arrow 52.

The contact pairs 117, 118 may be further arranged so that the signaldetection contacts 225, 226 are linearly aligned with respect to eachother and oriented in parallel to the axis of the arrow 52. Similarly,the reference contacts 227, 228 may be linearly aligned with respect toeach other and oriented in parallel to the axis of the arrow 52. Each ofthe cylindrical contacts 138 may be formed from a suitable electricallyconductive material with an exposed contact surface 222 of equal area.Each contact 138 may have an outer surface portion 139 for contactingthe skin after attachment of the sensor 134. Included in the bottomsurface 221 of the case may be a base portion 141 inwardly displacedfrom the outer surface contact portions 139 and concave transitionportions 142 extending between the base portion 141 and the oppositeedges of the outer surface portions 139 of each contact 138. Moldedcylindrical regions 143 of the base portion 141 may be situated betweenthe ends of each contact pair 117, 118 and act to form a continuation ofthe curve profile established by the transition portions 142. Each ofthe transition portions 142 may form a catenary curve to minimize theskin tensioning effect of the protruding contacts. In addition, thetransition portions 142 may eliminate any cavities between the contacts138 and the skin 61 which can accumulate sweat and thereby maintain ahigh impedance path in between the contacts 138.

Also included in the sensor embodiment 134 may be an attachmentinterface 156 shown in FIG. 14 having an adhesive interface surface 157for securing the interface 156 to the case 41 and an adhesive interfacesurface 158 for securing the case 41 to the skin 61 of the subject. Theinterface 156 defines a plurality of apertures 159 in the form of slotsdisposed to receive the contacts 138. Tab 91 is attached to one end 92of the top surface 157 of the interface 156. The sensor embodiment 134may be used in a manner similar to that described above for theembodiments 11 and 145.

A schematic diagram of a differential preamplifier recordingconfiguration that may be used with the biomedical sensor embodiments11, 134, 145 is shown in FIG. 15. The sensor embodiment 134 shownpreviously in FIGS. 12 and 13 is used for illustrative purposes. A highimpedance differential preamplifier 160 with differential inputs 161 mayinclude a positive input 162 and a negative input 163, and a referenceinput 164 may be connected to the contacts 138 of biomedical sensor 134using interconnections 166, 167, 168. The signal output 165 may be thearithmetic difference between the signal detected at contacts 225 and226 measured with respect to the reference contacts 227, 228. Theamplified output signal 165 may be interconnected with additionalassociated electronics 42 for wireless transmission of the detectedsignal.

FIGS. 16A-C show the representative plots of a simultaneous comparisonof sensor outputs between sensor embodiment 134 and a comparable two bar“reference free” sensor embodiment 234 attached in close proximity toone another on the skin 61 of a limb under conditions of: tapping theskin adjacent to sensor case (movement artifact plots 170; FIG. 16A);movement of the sensor in an electro-static field (electro-staticartifact plots 180; FIG. 16B); and friction of a garment on the skinadjacent to the sensor (garment friction plots 190; FIG. 16C). In eachof the FIGS. 16A-C, the sensor embodiment 134 is plotted on the uppertrace and a comparable two bar “reference free” sensor embodiment 234 isplotted on the lower trace.

In FIG. 16A, the movement artifact plots 170 display a time region 171containing multiple repetitions of tap induced artifact applied to theskin 61 in proximity to both sensor embodiments 134, 234. Over the timeregion 171, the sensor embodiment 134 indicates little or no artifactsignal present in the output. During the same time region 171, thesignal output from the two bar sensor embodiment 234 exhibits multiplelarge amplitude artifact responses, three of which are indicated 172.

In FIG. 16B, the electro-static artifact plots 180 display a time region181 containing multiple repetitions of a simultaneous flexion andextension limb displacement of the sensor embodiments 134, 234 appliedto the skin 61 with respect to a fixed electro-static field. Over thetime region 181, the sensor embodiment 134 indicates little or noartifact signal present in the output. During the same time region 181,the signal output from the two bar sensor embodiment 234 exhibitsmultiple large amplitude artifact responses, three of which areindicated 182.

In FIG. 16C, the garment friction plots 190 display a time region 191containing multiple repetitions of a simultaneous flexion and extensionlimb displacement of the sensor embodiments 134, 234 applied to the skin61 of a limb covered by clothing fabric. Over the time region 191, thesensor embodiment 134 indicates little or no artifact signal present inthe output. During the same time region 191, the signal output from thetwo bar sensor embodiment 234 exhibits multiple large amplitude artifactresponses, three of which are indicated 192.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A biomedical sensor for sensing phenomena of amuscle, comprising: a substrate; a plurality of conductive areasarranged to be spaced along a length of the substrate and configured assignal detection contacts each with a proximate, distinct respectiveassociated signal reference contact, forming a plurality of signaldetection/reference contact pairs spaced along the length, each one inthe plurality of signal detection/reference contact pairs being alignedin a direction transversely to the length and in parallel with respectto each other so that each signal detection/reference contact pairproduces movement induced artifacts in unison in response to relativemovement between the plurality of signal detection/reference contactpairs and the muscle; and a differential sensing circuit that receivesinput from each of the signal detection/reference contact pairs andprovides an output that is an arithmetic difference between the signalsdetected at detection contacts measured with respect to the referencecontacts, the reference contacts being directly electrically coupledtogether and to a reference input of the differential sensing circuit,and the detection contacts being provided at separate inputs to thedifferential sensing circuit, wherein each signal detection/referencecontact pair is configured to react in response to the relativemovement, wherein at least one common electrical component caused by therelative movement is cancelled out by the differential sensing circuit,and wherein the at least one common electrical component caused by therelative movement includes the movement induced artifacts that aresuppressed by the differential sensing circuit.
 2. The biomedical sensoras in claim 1, wherein the substrate is rigid.
 3. The biomedical sensoras in claim 1, further comprising: a framework coupled to the substrate,wherein the substrate is a body-directed electrically insulating bottomsurface of the framework.
 4. The biomedical sensor as in claim 1,wherein the plurality of signal detection/reference contact pairs arespaced apart so that the signal detection contact is aligned to be onthe same side of each pair.
 5. The biomedical sensor as in claim 1,wherein each of the conductive areas is formed in the shape of aconductive rod aligned in parallel on a plane of the substrate, andprojecting from the substrate.
 6. The biomedical sensor as in claim 1,wherein each of the conductive areas is formed in the shape of a discattached to and coplanar with the substrate.
 7. The biomedical sensor asin claim 1, wherein each of the conductive areas is formed in a shape ofa dome whose cross-sectional profile exhibits a concave curved shapeattached to and projecting from the substrate.
 8. The biomedical sensoras in claim 1, wherein each of the conductive contact areas are formedof substantially identical material composition, size and shape.
 9. Thebiomedical sensor as claim 1, wherein each of the conductive areas isformed from a noble metal.
 10. The biomedical sensor as in claim 1,wherein each of the conductive areas is formed from a conductiveplastic.
 11. The biomedical sensor as in claim 1, wherein the substrateis contoured in a concave shape to surround each of the conductiveareas.
 12. The biomedical sensor as in claim 3, wherein the frameworkincludes a protective enclosure retaining a multi-conductor cableelectrically connected to signal conditioning and amplifier circuitryinterconnected with a projecting portion of the contacts so that onlythe conductive areas are exposed.
 13. The biomedical sensor as in claim3, wherein the framework includes a protective enclosure retainingwireless communication circuitry, associated control circuitry, powersupply, and amplifier interconnected with a projecting portion of thecontacts so that only the conductive areas are exposed.
 14. Thebiomedical sensor as in claim 3, wherein framework includes anindicating marker aligned in parallel with a preferred orientation ofthe sensor with respect to a direction of signal propagation anddelineating a location of signal input contacts.
 15. The biomedicalsensor as in claim 1, wherein the substrate is attached to skin with adouble-sided adhesive membrane sheet with cut-out areas aligned with andmatching a perimeter of each of the conductive areas.
 16. A biomedicalsensor for sensing phenomena of a muscle, comprising: a fixed framework;an electrically insulating substrate rigidly coupled to the fixedframework; a plurality of conductive areas symmetrically arranged to bespaced along a length of the electrically insulating substrate andconfigured as signal detection contacts, each being associated with aproximate, distinct signal reference contact to form a plurality ofsignal detection/reference contact pairs spaced along the length, eachone in the plurality of signal detection/reference contact pairs beingaligned in a direction transversely to the length and in parallel withrespect to each other so that each signal detection/reference contactpair produces movement induced artifacts in unison in response torelative movement between the plurality of signal detection/referencecontact pairs and the muscle; and a differential sensing circuit coupledto the signal detection/reference contact pairs that provides an outputthat is an arithmetic difference between signals detected at thedetection contacts measured with respect to the reference contacts, thereference contacts being directly electrically coupled together and to areference input of the differential sensing circuit, and the detectioncontacts being provided at separate inputs to the differential sensingcircuit, wherein each signal detection/reference contact pair isconfigured to react in response to the relative movement, wherein atleast one common electrical component caused by the relative movement iscancelled out by the differential sensing circuit, and wherein the atleast one common electrical component caused by the relative movementincludes the movement induced artifacts that are suppressed by thedifferential sensing circuit.
 17. The biomedical sensor as in claim 16,wherein the plurality of signal detection/reference contact pairs arespaced apart so that the signal detection contact is aligned to be onthe same side of each pair.
 18. The biomedical sensor as in claim 16,wherein each of the conductive areas is formed in the shape of aconductive rod aligned in parallel on a plane of the substrate andprojecting from the substrate.
 19. The biomedical sensor as in claim 16,wherein each of the conductive areas is formed in the shape of at leastone of: a disc attached to and coplanar with the substrate and a domewhose cross-sectional profile exhibits a concave curved shape attachedto and projecting from the substrate.
 20. The biomedical sensor as inclaim 1, wherein the proximal arrangement is a side-by-side arrangementof the signal detection contact with the respective reference contact ofeach signal detection/reference contact pair, each signal detectioncontact being associated on a one-to-one basis with a differentreference contact in each signal detection/reference contact pair.